In a 1790 letter to the Touro Synagogue in Rhode Island, President George Washington promised that America's Jews would always enjoy the full rights, privileges, and protections of U.S. citizenship. Since that auspicious beginning, Jews have flourished in America as they have nowhere else in the modern world. The last half-century in particular has been a kind of "Golden Age" for American Jews, during which they have achieved unprecedented levels of social acceptance, professional success, and political and cultural influence. But has all that come at the expense of Jewish distinctiveness?...